Lord Grezington
New Member
Hi there
I am designing an H-Bridge and learning a lot along the way, I have come up with a few questions I am hoping some of you can help me answer.
If I am sending a 5V 20Khz PWM signal from the PIC into a Mosfet, and to ensure that I do get the full PWM into the Gate of the Mosfet I want to use a Pull up/Down resistor. I am fairly new to this, but if I use a Pull down, I loose a few volts at the top end of the pwm, and if I pull up, the voltage never really gets back down to ground. And I will get a larger voltage drop when using a lower value resistor. But also, lower value resistors absorb more power and hence make the circuit less efficient (I am not really worried about this). Is this theory correct? And what is better for PWM, up or down?
Also, if I Use a large value resistor the PIC has easily enough "Power" to switch and can do this fairly quickly, and, if I have a low value resistor (if it is too low) it will take much longer to switch (if it switches at all). Now, the problem I have is that the Mosfet switching around 20 KHz (and the motor wires PWM'ing it to 24Vdc) creates a lot of EMC. I am worried that this EMC is going to send noise spikes into the control electronics and manipulate the 5V PWM. I am trying to keep the board to a double layer (so no ground plane). So now a lower value resistor is recommended as there is less chance of any noise pulling the pwm up or down?
So to conclude,
Is it better to pull up or down?
What calculations can I use to specify the pull up/down resistor in an electrically noisy environment?
Should I use some capacitors to filter out some of the EMC? if yes, what calculations can I use to make the specification?
P.S - I have also seen on some bridges that people use a 1K resistor in series with the Mosfet. Is my theory correct when I say that all this is going to do is create a voltage drop across the resistor and hence pull less voltage onto the gate, hence taking longer to charge the gate capacitor and increasing switching time?
Thanks in advance
Graham
I am designing an H-Bridge and learning a lot along the way, I have come up with a few questions I am hoping some of you can help me answer.
If I am sending a 5V 20Khz PWM signal from the PIC into a Mosfet, and to ensure that I do get the full PWM into the Gate of the Mosfet I want to use a Pull up/Down resistor. I am fairly new to this, but if I use a Pull down, I loose a few volts at the top end of the pwm, and if I pull up, the voltage never really gets back down to ground. And I will get a larger voltage drop when using a lower value resistor. But also, lower value resistors absorb more power and hence make the circuit less efficient (I am not really worried about this). Is this theory correct? And what is better for PWM, up or down?
Also, if I Use a large value resistor the PIC has easily enough "Power" to switch and can do this fairly quickly, and, if I have a low value resistor (if it is too low) it will take much longer to switch (if it switches at all). Now, the problem I have is that the Mosfet switching around 20 KHz (and the motor wires PWM'ing it to 24Vdc) creates a lot of EMC. I am worried that this EMC is going to send noise spikes into the control electronics and manipulate the 5V PWM. I am trying to keep the board to a double layer (so no ground plane). So now a lower value resistor is recommended as there is less chance of any noise pulling the pwm up or down?
So to conclude,
Is it better to pull up or down?
What calculations can I use to specify the pull up/down resistor in an electrically noisy environment?
Should I use some capacitors to filter out some of the EMC? if yes, what calculations can I use to make the specification?
P.S - I have also seen on some bridges that people use a 1K resistor in series with the Mosfet. Is my theory correct when I say that all this is going to do is create a voltage drop across the resistor and hence pull less voltage onto the gate, hence taking longer to charge the gate capacitor and increasing switching time?
Thanks in advance
Graham